DESCRIPTION (Adapted from applicant's description): This prospective case-control study will determine whether the practice of breast-feeding during pregnancy is associated with a change in the composition of early milk and a subsequent decrease in milk volume during the next lactation. Because social taboos in the US discourage women from openly breast-feeding through pregnancy, the study can be more efficiently carried out in Peru. 168 multiparous women living in Lima, Peru will be enrolled. Eighty-four women will be identified who are breast-feeding during the third trimester of pregnancy (cases); 84 controls, who have not breast-fed past the first month of pregnancy will be identified after giving birth and will be matched for interbirth interval and sex of the newborn. Intensity of breast-feeding during the last trimester of pregnancy will be determined for the cases. D2 milk composition and volume will be determined. If breast-feeding during pregnancy alters the subsequent lactation, then it would be hypothesize that case infants; intakes of secretory IgA, IgG, total proteins, fat, lactose, and vitamin A would significantly differ than that of controls. Milk volume intake at 1 mo would be significantly decreased. The results of this study will provide the first information on the biological consequences of lactation-pregnancy overlap in humans, essential information needed to understand the health implications of this widespread but little mentioned practice.